De-flaking some tests in virtual/gpu/fast/canvas

LayoutTests/fast/canvas/canvas-zoom.html

+<script src="resources/asyncpaintguard.js"></script>
 <style>
     canvas { border: solid green;
              zoom: 2; }
 </style>
 <p>
     These should be four green hollow boxes with dimensions 600x300, 100x300, 600x100, 100x100.
 </p>
 <!-- 300x150 -->
 <canvas id="canvas"></canvas>
 <!-- 50x150 -->
 <canvas id="canvas" width="50"></canvas>
 <!-- 300x50 -->
 <canvas id="canvas" height="50"></canvas>
 <!-- 50x50 -->
 <canvas id="canvas" width="50" height="50"></canvas>